Accurate Determination of 1 H‐ 15 N Dipolar Couplings Using Inaccurate Settings of the Magic Angle in Solid‐State NMR Spectroscopy

Measuring Dipolar Order Parameters in Nondeuterated Proteins Using Solid-State NMR at the Magic-Angle-Spinning Frequency of 100 kHz

Proteins are dynamic molecules, relying on conformational changes to carry out function. Measurement of these conformational changes can provide insight into how function is achieved. For proteins in the solid state, this can be done by measuring the decrease in the strength of anisotropic interactions due to motion-induced fluctuations. The measurement of one-bond heteronuclear dipole-dipole coupling at magic-angle-spinning (MAS) frequencies >60 kHz is ideal for this purpose. However, rotational-echo double resonance (REDOR), an otherwise gold-standard technique for the quantitative measurement of these couplings, is difficult to implement under these conditions, especially in nondeuterated samples. We present here a combination of strategies based on REDOR variants ϵ-REDOR and DEDOR (deferred REDOR) and simultaneously measure residue-specific ¹⁵N-¹H and ¹³C_α-¹H_α dipole-dipole couplings in nondeuterated systems at the MAS frequency of 100 kHz. These strategies open up avenues to access dipolar order parameters in a variety of systems at the increasingly fast MAS frequencies that are now available.

Ultrafast Magic Angle Spinning Solid-State NMR Spectroscopy: Advances in Methodology and Applications

Solid-state NMR spectroscopy is one of the most commonly used techniques to study the atomic-resolution structure and dynamics of various chemical, biological, material, and pharmaceutical systems spanning multiple forms, including crystalline, liquid crystalline, fibrous, and amorphous states. Despite the unique advantages of solid-state NMR spectroscopy, its poor spectral resolution and sensitivity have severely limited the scope of this technique. Fortunately, the recent developments in probe technology that mechanically rotate the sample fast (100 kHz and above) to obtain "solution-like" NMR spectra of solids with higher resolution and sensitivity have opened numerous avenues for the development of novel NMR techniques and their applications to study a plethora of solids including globular and membrane-associated proteins, self-assembled protein aggregates such as amyloid fibers, RNA, viral assemblies, polymorphic pharmaceuticals, metal-organic framework, bone materials, and inorganic materials. While the ultrafast-MAS continues to be developed, the minute sample quantity and radio frequency requirements, shorter recycle delays enabling fast data acquisition, the feasibility of employing proton detection, enhancement in proton spectral resolution and polarization transfer efficiency, and high sensitivity per unit sample are some of the remarkable benefits of the ultrafast-MAS technology as demonstrated by the reported studies in the literature. Although the very low sample volume and very high RF power could be limitations for some of the systems, the advantages have spurred solid-state NMR investigation into increasingly complex biological and material systems. As ultrafast-MAS NMR techniques are increasingly used in multidisciplinary research areas, further development of instrumentation, probes, and advanced methods are pursued in parallel to overcome the limitations and challenges for widespread applications. This review article is focused on providing timely comprehensive coverage of the major developments on instrumentation, theory, techniques, applications, limitations, and future scope of ultrafast-MAS technology.

Determination of methyl order parameters using solid state NMR under off magic angle spinning

Quantification of dipolar couplings in biological solids is important for the understanding of dynamic processes. Under Magic Angle Spinning (MAS), order parameters are normally obtained by recoupling of anisotropic interactions involving the application of radio frequency pulses. We have recently shown that amide backbone order parameters can be estimated accurately in a spin-echo experiment in case the rotor spinning angle is slightly mis-calibrated. In this work, we apply this method to determine methyl order parameters in a deuterated sample of the SH3 domain of chicken α-spectrin in which the methyl containing side chains valine and leucine are selectively protonated.